This invention relates to a defect detector circuit for an inductive load driving circuit for detecting that a circuit for driving an inductive load such as a coil or the like of an electromagnetic valve becomes defective.
An electromagnetic driving device for converting an electric signal to a mechanical operation has been heretofore used. For example, an electromagnectic fuel injection valve (hereinbelow termed "an electromagnetic valve") which operates on the basis of a command signal from a control circuit to inject fuel has been disclosed as a component of an electronic control type fuel injection device of an internal combustion engine for an automobile in Japanese Patent Application Laid-open No. 59-150935.
FIGS. 8 to 11 indicate a prior-art driving circuit for driving an electromagnetic valve and a defect detector circuit for detecting the defect of the driving circuit as described above. In FIG. 8, reference numeral 1 denotes a calculating circuit which inputs a signal representing various operating states of an internal combustion engine such as an intaken air amount, calculates a fuel injection amount and outputs a command signal S.sub.1 for driving an electromagnetic valve. Numeral 2 denotes a switching circuit which receives the command signal S.sub.1 from the calculating circuit 1, and conducts or nonconducts a current of a coil 3 of the electromagnetic valve, and which consists of a resister 2a connected at its one end to the output of the calculating circuit 1, and a transistor 2b the base of which is connected to the other end of the resistor 2a, the collector of which is connected to one end of the coil 3, the other end of the coil 3 being connected to a battery voltage V.sub.B, and the emitter of which is connected to ground to operate as a switching element. Numeral 4 denotes a surge absorber circuit for absorbing a surge generated when the current of the coil 3 is interrupted. The surge absorber circuit 4 consists of a resistor 4a, connected at one end thereof to the collector of the transistor 2b, and a capacitor 4b connected in series between the other end of the resistor 4a and ground. a driving circuit 5 is composed of the switching circuit 2 and the coil 3. Numeral 6 denotes a defect detector circuit for detecting a defect in the coil 3 and/or the switching circuit 2. The detect detector circuit consists of a resistor 6a connected at one end thereof to the collector of the transistor 2b, a resistor 6b connected at one end thereof to a power source voltage V.sub.cc, a transistor 6c the base and collector of which are respectively connected to the other ends of the resistors 6a and 6b, and the emitter of which is connected to ground to operate as a switching element, and an exclusive OR circuit 6d which receives a signal S.sub.3 from the collector of the transistor 6c and the command signal S.sub.1 from the calculating circuit 1, and outputs a defect signal S.sub.4. Numeral 7 denotes a defect indicator circuit for indicating a defect on the basis of a defect signal S.sub.4 from the defect detector 6. The defect indicator circuit 7 consists of a resistor 7a connected at one end thereof to the output terminal of the exclusive OR circuit 6d, a lamp 7b connected at one end thereof to the battery voltage V.sub.B, and a transistor 7c the base and collector of which are respectively connected to the other ends of the resistor 7a and the lamp 7b, and the emitter of which is connected to ground to operate as a switching element.
The operation of the prior-art defect detector circuit constructed as described above will now be described. The operation of the detect detector circuit under normal conditions when no defect exists in the coil 3 and the switching circuit 2 will first be described with reference to FIG. 9. The calculating circuit 1 outputs, as shown in FIG. 9, a command signal S.sub.1 comprising a pulse train signal based on fuel injection amount. The transistor 2b of the switching circuit 1 receives the command signal S.sub.1 to either turn on or off, thereby to either conduct or nonconduct current through the coil 3, respectively. Specifically, when the command signal S.sub.1 is an "H" level (i.e., at the power source voltage Vcc), the transistor 2b is turned on so that a current flows through the coil 3, and the collector potential S.sub.2 of the transistor 2b becomes an "L" level (the ground potential). Similarly, when the transistor 6c is turned off, its collector potential S.sub.3 becomes the "H" level. Since the potential S.sub.3 an the command signal S.sub.1 are at the "H" level at the same time, the output S.sub.4 of the exclusive OR circuit 6d becomes the "L" level. However, when the command signal S.sub.1 returns to the "L" level, namely, when the command signal S.sub.1 is switched from the "H" level to the "L" level (the ground potential), the transistor 2b is, in turn, turned off, a causing a surge to be generated in the coil 3 due to the self-induction electromotive force thereof. The surge is absorbed to a certain degree by the surge absorber circuit 4. However, a surge voltage Su as shown in FIG. 9 is generated. Thus, since the surge decreases to wards the battery voltage V.sub.B as a function of time, the potential S.sub.2 is actually the battery voltage V.sub.b or higher, while the command signal S.sub.1 remains at the "L" level. The transistor 6c is, in turn, turned on, and the collector potential S.sub.3 of the transistor 6c becomes the "L" level. Since the potential S.sub.3 and the command signal S.sub.1 together become the "L" level, the output S.sub.4 of the exclusive OR circuit 6d remains at the "L" level. Therefore, since the output S.sub.4 of the defect detector 6 is at the "L" level even when the command signal S.sub.1 is at the "H" or "L" level, the transistor 7c of the defect indicator circuit 7 is in a normally open state, the lamp 7b is not lit to thereby indicate that there is to defect.
The case where the transistor 2b of the switching circuit 2 becomes defective to indicate a normal-open state (a wire disconnection defect occurs) will be described with reference to FIG. 10. Since the collector potential S.sub.2 of the transistor 2b is always at the battery voltage V.sub.B as shown in FIG. 10, the transistor 6c is necessarily always in a turned on state, the collector potential S.sub.3 of the transistor 6c is always at the "L" level, whereby the output S.sub.4 of the exclusive OR circuit 6d becomes a pulse train signal synchronized with the command signal S.sub.1 as shown in FIG. 10. Since the transistor 7c of the defect indicator circuit 7 is turned on so that the lamp 7b is lit when the output S.sub.4 is the "H" level, the lamp 7b repeatedly flashes synchronously with the command signal S.sub.1 to indicate that there is a defect.
The cases where the transistor 2b of the switching circuit 2 is always in the turned on state (i.e., a shortcircuit defect occurs) and the coil 3 is disconnected to be defective will be described with reference to FIGS. 11. In this case, since the collector potential S.sub.2 of the transistor 2b of the switching circuit 2 always indicates the ground potential, the collector potential S.sub.3 of the transistors 6c always becomes the "H" level. Thus, the output S.sub.4 of the exclusive OR circuit 6d becomes a pulse train signal which is the command signal S.sub.1 inverted as shown in FIG. 11. Therefore, the lamp 7b flashes similarly to the above-described defect to indicate that there is a defect.
Since the lamp 7b does not light at all when the switching circuit 2 and the coil 3 are not defective and the lamp 7b flashes when there is a defect, a driver can immediately know that the cause of the defect exists in the driving circuit 5 during the time that the internal combustion engine is not operated if the lamp 7b is provided on an instrument panel of an automobile.
According to the above-described prior-art defect detector circuit 6, the wire disconnection and the shortcircuit defect of the transistor 2b of the switching circuit 2 can be detected. However, defects such as a partial shortcircuit @shortcircuit between wirings) or the deterioration of the coil 3, a wire disconnection, the shortcircuit or the deterioration of the resistor 4a or the capacitor 4b of the surge absorber circuit 4 can not be detected, since the timing of the opening and the closing the transistor 6c and the output of the exclusive OR circuit 6d are entirely the same as those of the cases of the normal time in which the waveform of the surge voltage Su slightly changes even if the defect occurs. If such a defect occurs, the surge absorbing characteristic of the coil 3 is displaced from the set characteristic, and the response delay time (due to the self-induction electromotive force of the coil) from when the command for interrupting the current of the oil 3 by the command signal S.sub.1 is outputted to when the electromagnetic valve actually stops operating is displaced with respect to the response delay time at the normal time. Thus, even if the time for opening the valve inputted to the calculating circuit 1 is accurate, the actual valve opening time contains an error. Thus, there arises a drawback that an accurate fuel amount is not supplied to the internal combustion engine, whereby the operating performance of the engine is deteriorated.